Image formation

Question 1

How do we analyse a FID?

Answer 1

You apply fourier transform, gives you a spectrum of the resonance frequencies in the sample.

Question 2

Encoding spatial information

How do you do it?

Describe sequence of of simple 1D imaging experiment

How do you get the image?

Answer 2

Frequency of MR signal is determined by strength of magnetic fiel. If you apply a magnetic field gradient across the sample, you apply a position dependent change in resonant requency. w = y(B₀ + G_r*x)

• Excite with RF pulse
• Apply field-gradient to encode spatial distribution into larmour frewuency
• sample the FID, which is now composed of a mix of frequencies.

Fourier transform: components of signal have frequency determined by location in sample. Amplitude of each frequency component depends on the number of moleculesx in the sample at the location perpendicular to gradient axis (1D image profile).

Question 3

Readout Gradient

What effect does it have on magnetisation in x,y plane?

Why does it need to be modified?

How do you modify it?

What’s the name and components of the sequence to do this?

Answer 3

Spins in area of higher magnetic field (due to gradient) process faster in the x-y plane. Spins in area of lower magnetic field process slower.

Needs to be modified to collect ‘negative time’ data from before the RF pulse to improve FT.

There is a dephasing effect casued by spins are processing at different rates depending on their position in the magnetic field. You get round this by applying gradient in the opposite direction immediatley after to reverse the dephasing, magnetisation of spins converging in x-y plane. When duration of positive gradient is same as initial, all spins are converged. Continue on past that.

(GRADIENT ECHO).

• RF
• negative gradient on
• positive gradient on for twice as long as negative, start acquiring FID as spins reconverge.

Question 4

How do you encode a second spatial dimension?

Pulse sequence?

Answer 4

Phase encode gradient applied orthogonally to readout gradient. Adds a position dependent phase change to sample. Stronger gradient = larger dephasing. Parts of sample where phases line up give signal, hence by changing the amount of dephasing, you are probing the amount of each spatial frequency present in the material. (water tank anaology)

1. Apply PE gradient orthogonal to readout gradient
2. Repeat sequence whilst arraying the amplitude of the PE gradient
3. PE gradient modulates the phase of the accquired signal
4. 2D FT converts into image

Question 5

Effect of gradients on magnetisation, visualised in k-space

Answer 5

• Readout gradient maps out a row of points
• The amplitude of the PE gradient takes you to a particular row (e.g. y=-1, y=3).
• You repeat the sequence (readout gradient + PE grad at a partociar amplitude). Each repetition accquires data along a different line of k-space. Arraying over different PE amplitudes to sequentially map out k-space. Filling in 1 row at a time.

Question 6

How do you get 1D profile in

x-direction

y-direction

Answer 6

x-direction - FT across a row of k-space

y-direction - FT across a collumn of k-sapce

Question 7

How do you encode 3rd dimension?

What controls slice position?

What controls slice thickness?

Answer 7

Slice selection gradient applied during RF pulse to change resonant frequency of sample’s magnetisation. RF pulse only excites regions of sample that have resonant frequency within bandwidth of pulse.

Frequency of RF pulse determines slice position

stronger gradient gives thinner slice

Question 8

Complete grafient-echo imagin sequence

All to the way to getting the image out.

Answer 8

• RF excitation
• Slice selection gradient applied along slice direction during RF, only exciting the slice on resonance/within the bandwidth of the RF.
• Readout gradient to add position dependent dephasing to signal. NB// initial negative grad and then positive grad for twice as long to rephase signals (flip 180 and Mxy vectors converge) and form gradient echo.
• Phase encode gradient applied orthogonal to readout at same time as readout gradient to apply position dependent phase change to signal in second direction.
• Accquire for duration of positive readout radient, record the echo FID.
• REPEAT sequence, arraying at different PE gradent amplitudes to map out k-space. Each repetition probes a different row of k-space
• 2D FT to get image.